Optical inspection is commonly used in semiconductor device manufacturing to detect defects on the surface of a wafer, such as contaminant particles, scratches and unremoved portions of material layers. Defects can cause device failures, thus substantially reducing the process yield. Therefore, careful inspection is required to verify the cleanliness and quality both of unpatterned wafers and of patterned wafers at various stages of the manufacturing process.
A common method for inspecting semiconductor wafers is to scan a laser beam over the wafer surface and measure the light scattered from each point on which the beam is incident. An exemplary inspection system, based on dark-field scattering detection, is described in U.S. patent application Ser. No. 10/511,092, cited above. Other optical inspection systems are described, for example, in U.S. Pat. Nos. 6,366,690, 6,271,916, 6,538,730, 6,392,793 and 5,909,276, whose disclosures are incorporated herein by reference.
In some applications, the optical inspection system uses two different light beams. For example, U.S. Pat. No. 4,191,476, whose disclosure is incorporated herein by reference, describes a system and method in which two patterns of illumination are formed independently on a photosensitive screen by means of light of two different wavelengths. The patterns are formed in the same manner, and each results from irradiation of the screen with light derived from a coherent source and consisting of two interfering beams, one of which is constituted by light scattered from the surface under inspection and imaged on to the screen. A video signal is derived from the screen representing the spatial variations in the sum of the intensities in the two patterns.
Another inspection system comprising dual-beam illumination is described in U.S. Pat. No. 6,825,924, whose disclosure is incorporated herein by reference. The system comprises an illuminator comprising a dual peak wavelength tube, which is a light source emitting two different colors of light. Each colored light has a narrow wavelength range, with a peak wavelength at a respective one of two complementary colors. The system comprises supporting means for supporting a substrate having a surface on which predetermined patterns are finely formed. The surface is illuminated at a predetermined angle with the light emanating from the illuminator, with the surface functioning as a diffraction grating. The system determines whether the pattern surface on the substrate is deformed due to defocusing as a result of said light being diffracted by the finely patterned surface.
U.S. Pat. No. 4,643,569, whose disclosure is incorporated herein by reference, describes a dual-beam laser inspection system comprising a scanner, which repetitively scans a line with first and second angularly displaced, synchronized scans. The system processes data received from each of the dual scanning beams in parallel, thereby reducing the data processing rate in comparison to a single-beam laser inspection apparatus operating at the same inspection rate.